This invention relates to a method of forming ceramic parts having internal structure such as ceramic gas turbine blades and nozzles with internal cooling air passages.
To withstand the high gas temperatures generated in the combustor of gas turbine propulsion engines, single crystal, super-alloy turbine blades and nozzles, just downstream of the combustor, employ internal cooling channels and/or thermal barrier coatings. At such high operating temperature, turbine engines can generate high thrust with low fuel consumption and pollutant levels. Over the last two decades, is significant efforts have been devoted to the improvement of high temperature properties of single crystal super-alloys and their casting technology to satisfy the increasing demand for high performance turbine engines. Although the technology of manufacturing turbine engine components with cooling channels is well established, the cost associated with these types of parts is very high because of the low yield and the need of a thermal barrier coating. Normally, the part is fabricated from a lost wax process. In that process, a wax pattern having the shape of the final part and a ceramic core of the cooling channel structure is manufactured, and the casting mold is fabricated from repeatedly dipping the wax pattern into ceramic slurries. During the removal of wax pattern through melting, very often the mold cracks because of the expansion of the wax, reducing the yield of the process.
Silicon nitride ceramics are known for their excellent strength, toughness, creep rupture resistance and the overall capability to withstand high temperatures. Using a standard processing method like slip-casting, bisque machining, injection molding, iso-pressing, or gel-casting, they can be manufactured into complex shapes. Indeed, silicon nitride ceramics have been manufactured into uncooled turbine blades and nozzles that match or exceed the performance of those made from single crystal super-alloys with thermal barrier coating and cooling channels. Because cooling from the engine is not needed to cool the blades, the engine operates with much higher efficiency.
Without the cooling and thermal barrier coatings, advanced silicon nitride parts can operate in gas temperatures up to 1400xc2x0 C. However, there is a strong desire among the engine manufacturers to further increase the turbine engine gas temperature to 1500-1600xc2x0 C. for high efficiency where even the best silicon nitride would have difficulty meeting the mechanical property requirements. At such gas temperatures, either cooled metallic components or uncooled silicon nitride components cannot survive the environment.
Accordingly, there is a need for a method for forming ceramic parts with internal cooling air structures.
An object of the present invention is to provide a method for forming a ceramic part with an internal structure.
Another object is to provide a ceramic article having an internal structure.
The present invention achieves these objectives by providing a method for forming a ceramic part with an internal structure that includes aqueous slip-casting and a polystyrene foam insert. An aqueous slurry is poured over the insert which is shaped to produce the desired internal structure upon being encased by the slurry to form a green body. The insert is then removed by dissolving it in trichloroethylene. Because the polystyrene foam does not expand when exposed to trichloroethylene and rapidly dissolves, build up of shrinkage stress in the green body is avoided. The green body is then sintered to produce the ceramic part with an internal structure.
These and other objects, features and advantages of the present invention, are specifically set forth in, or will become apparent from, the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawing.